Olaf's Attack Speed is increased by 1% for each 1% of his Health he is missing.

Undertow

Q

Olaf throws an axe into the ground at a target location, dealing damage to enemies it passes through and slowing their Movement Speed. If Olaf picks up the axe, the ability's cooldown is reduced by 4.5 seconds.

Vicious Strikes

E

Olaf's Attack Speed is increased, he gains Life Steal and has increased healing from all sources based on how much Health he is missing.

Reckless Swing

W

Olaf attacks with such force that it deals true damage to his target and himself, refunding the Health cost if he destroys the target.